Olefinic hydrocarbons are frequently used in many industrial applications, including the manufacture of polymers such as polyethylene, as drilling mud additives, and as intermediates for the production of detergents. There exist many industrial processes for the manufacture of olefins. However, many of these processes produce hydrocarbon streams that are mixtures of olefinic hydrocarbons, paraffinic hydrocarbons and often other impurities such as oxygenates and aromatics.
One such process for the production of olefins is the partial dehydrogenation of paraffinic hydrocarbons, which produces olefinic hydrocarbons and some unreacted paraffinic hydrocarbons. Another such process is the dehydration of alcohols, which produces an olefinic hydrocarbon composition as well as paraffinic hydrocarbons and unreacted alcohol feedstock.
A further industrial process which may be used for the manufacture of olefins is the so-called Fischer-Tropsch (FT) hydrocarbon synthesis process. The feedstock for this process is syngas, a mixture of carbon monoxide and hydrogen, which is generally inexpensive and can be readily derived from natural gas, coal, coke, and other carbonaceous compounds.
Unfortunately, however, the FT process is not always selective towards the production of olefins. The product stream derived from the Fischer-Tropsch process typically contains a mixture of olefins, saturated hydrocarbons, aromatics and oxygenates. While reaction conditions and catalysts can be tuned to manufacture streams rich in a desired species (e.g. olefinic hydrocarbons), the FT product stream can also contain a significant percentage of the other types of compounds produced by the FT process (e.g. paraffinic hydrocarbons).
Clearly, olefinic hydrocarbon compositions produced from the processes mentioned hereinabove should be purified, by separation of the desired olefinic products from the other products produced, before they are sold commercially. However, conventional distillation techniques are frequently inadequate to separate olefinic hydrocarbons and paraffinic hydrocarbons which have similar molecular weights since they frequently have closely related boiling points.
Various processes for separating olefinic hydrocarbons from paraffinic hydrocarbons are taught in the prior art. One such process involves the use of an ionic liquid composition comprising a metal salt dissolved or dispersed in the ionic liquid solution for the separation of olefinic hydrocarbons from paraffinic hydrocarbons wherein the metal salt/ionic liquid forms a complex with the olefinic hydrocarbon. The separation process can be described by the following general scheme: firstly, a two-phase composition of the olefin/paraffin composition and the metal salt/ionic liquid composition is formed; secondly, the olefins are absorbed and complexed with the metal salt/ionic liquid composition; thirdly, the resultant paraffinic composition is separated from the metal salt/ionic liquid composition; and finally, the olefins are isolated from the metal salt/ionic liquid composition by desorption at increased temperature and/or reduced pressure.
The use of membranes for the separation of olefinic hydrocarbons from paraffinic hydrocarbons has been achieved with varying degrees of success. However, membrane separation processes have only regularly been reported for the purification of gaseous olefinic hydrocarbon feedstocks having a narrow distribution of carbon numbers, typically a single carbon number, e.g. C2 or C3.
Despite there being processes available for separating liquid olefinic hydrocarbons and liquid paraffinic hydrocarbons, there is still a need to provide an improved process for the separation of olefinic hydrocarbons from paraffinic hydrocarbons. In particular, there is a need to provide a process which will not only provide olefinic compositions of sufficient purity to be commercially useful but which will also be highly energy efficient. It would also be desirable to provide a process which is capable of separating olefinic hydrocarbons from paraffinic hydrocarbons in a liquid feedstock composition having a broad spectrum of average carbon numbers (e.g. C5-C40)